Turbine erosion protective device



Sdtilz iatented Dec. 4, 1952 ice 3,066,912 TURBHNE EROSIUN PROTECTIVEDEVICE George W. Scheper, Jr., Schenectady, NX., assigner to GeneralElectric Company, a corporation of New York Filed Mar. 28, 1961, Ser.No. 98,955 6 Claims. (Cl. 253-76) This invention relates to an improvedarrangement for reducing erosion in a turbo-machine wherein the motivefluid contains erosive matter, and particularly for protecting thehighly stressed moving parts of the turbo machine. More specifically,the invention relates to the removal or control of erosive fly-ash in acoal-burning gas turbine with emphasis on protecting buckets of theearly stages thereof.

It is well known that solid or liquid particles in the motive fluid of aturbine, such as a steam or gas turbine, can cause erosion, and thatthis erosion increases as an exponential function of the motive fluidvelocity through the turbine. This is one of the major design problemswhich has delayed the advent of a successful coal-burning gas turbine.Since coal is cheaper, it would be preferable to the common gas turbinefuels., Le., residual or distillate petroleum oils or natural gas.However, tests with experimental coal-burning gas turbines haveindicated that concentrated erosion occurs at locations near the rootsof the blade members. This is particularly serious in thehighly-stressed rotating turbine buckets, and even more serious in thefirst-stage turbine bucketwheel, which is subjected to a Very hightemperature of the motive fluid.

Accordingly, one object of the present invention is to remove or controlerosive particles in the motive fluid of a turbo-machine.

A more specific object is to remove or control ily-ash or othersuspended solids in a coal-burning gas turbine.

A still more specific object of the invention is to protect the bucketsof the early stages in a coal-burning gas turbine, while keepingaerodynamic losses in the turbine to a minimum.

The subject matter of the invention is particularly pointed out anddistinctly claimed in the concluding portion of the specication. Theinvention, however, both as to organization and method of practice,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing in which:

FIG. 1 is an elevation drawing, in section, of the bladed portion of acoal-burning gas turbine, and

FIG. 2 is a schematic plan View of the first turbine stage.

Briefly stated, the invention is practiced by separating an early stagenozzle, such as the first-stage nozzle ring, axially from an early stagebucket, such as the first-stage bucket wheel, by more than apredetermined critical minimum spacing. The spacing is chosen so as toeffect the outward movement of the solid particles by centrifugal forceaway from the critical root portion of the turbine bucket and to theoutside diameter of the flow annulus where they may be removed.

In order to reduce aerodynamic losses engendered by the increased axialspacing and to further control erosion, the number of turbine stages isincreased and at least one side wall of the flow annulus betweenfirst-stage nozzle and rst-stage bucket may be arranged to rotate.

Referring now to FIG. l of the drawing, a portion of the turbine outercasing 1 forms the outside wall of an annular flow duct or flow annulussymmetrical with the turbine rotor axis (not shown) and commencing atthe turbine inlet 2, where the flow direction is indicated by the arrow.The casing 1 serves as the means to support a number ofcircumferentially spaced, radially extending nozzle partitionsdesignated 3, 4, 5, 6, 7 which serve to re-direct the motive fluid ineach of the tive turbine stages. Each of the nozzles 3-7 has vaneportions 31a-7a disposed in the motive fluid path, and dovetailed baseportions 31u-7b disposed in mating slots in casing i, to hold thenozzles in place. The tips of nozzle partitions 4-6 do not includeshrouds in this embodiment, but the radially innermost parts of nozzlepartitions 3 and 7 include circumferentially extending shroud portions3c, 7c respectively, to serve as the inner wall of the dow annulus. Theforegoing comprise the stationary blade portions of the turbine.

Mounted on a rotating shaft (not shown) are a number of turbine wheelsti, upon which are disposed turbine buckets 9, le", il, 12, 13, servingto extract the energy from the motive fluid. A turbine bucket togetherwith the nozzle partition ahead of it is called a stage Thus the firststage includes nozzle 3 and bucket 9; the second stage includes nozzle 4and bucket 16 and so forth. The turbine buckets have vane portions 9othrough ia which are exposed to the motive fluid, and dovetailed baseportions Qb through 13b mounted in mating slots on turbine wheels 8.Buckets 9-12 also include platform ledges 14 which extend axially towardone another and also abut circumferentially to form a rotating innerwall for the motive iiuid flow path. Platform ledges 1d also substituteas sealing surfaces forming close clearances with the unshouded statorpartitions 4-6.

The tips of the buckets and nozzle partitions may also overlap beyondthe roots of the vane portions of adjacent members, and the platforms ofboth nozzle partitions and buckets may be provided witherosion-resistant inserts 15 as set forth in my U.S. Patent 3,030,07lissued April i7, 1962, and assigned to the assignee of the presentapplication. These inserts i5 serve as impingernent surfaces for motivefluid and particles passing under the blade tips.

Separating the first-stage nozzle blade 3 and the firststage turbinebucket 9 is an axial gap of a minimum distance a, to be furtherspecified. In order to provide a rotating inner side wall for the flowannulus across this axial gap, a number of dummy buckets lo areprovided. These are bucket bases with platform portions but no vaneportions. The dummy ybuckets lo abut circumferentially -to provide arotating inner side wall.

The outer casing wall 1 defines a `contoured fly-ash. extraction port 17leading to an annular collecting chamber 18. Extraction port 17 as shownis a circumferential groove in the casing wall 1. A fly-ash removal pipe19 connects with annular collecting chamber 18 and leads to a fly-ashcollecting hopper or cyclone separator Ztl. The return line 2l isconnected to a restricted inlet passageway 22 at a downstream or lowerpressure location in the turbine, shown here as emptying into the tlowpath between bucket 9 and nozzle partition 4. The return line 2l servesto conserve the heat and pressure energy in the extracted motive fluidby returning it to the cycle.

The axial distance between the exit edge 3d of nozzle partition 3 andthe downstream edge 17a of extraction port 17 is denoted by a. Dimensiona must be a minimum critical dimension in order for fly-ash particlesleaving nozzle partition 3 at the radially innermost point thereof topass out through extraction port 17 before hitting the edge 17a of theport. The path of a particle is symbolized by the dotted line 23,although it is understood that this is not a true path since theparticle has axial, tangential and radial components of velocity all atthe same time. The minimum axial spacing a is affected both by the innerand outer radii of the flow annulus taken from the rotor axis and by thetangential component of motion given to the fly-ash particle as itleaves the exit edge 3d of the nozzle partition 3. This is seen moreclearly by reference to FIG. 2 of the drawing, which is a schematic planview showing two nozzle partitions 3 having trailing edges 3d anddirecting the gas toward two buckets 9. Axial spacing a is shown toextend between trailing edge 3a.' of the nozzle partitions and thedownstream edge 17a of extraction 17.

The tangential component given to the ily-ash particles is determined bythe angle ai which is the angle of the nozzle partition exit edge fromthe tangential direction measured at the radially innermost part of thenozzle partition vane exposed to the motive fluid. This minimum requiredaxial spacing can be determined by the following formula:

062125- tan OLA/(RO/RQZ-l where:

Ri is the inner radius of the llow annulus measured `from the rotor axisat the location of the nozzle partition trailing edge,

R is the outer radius of the ow annulus at the point where it is desiredthat the particles strike, and

ai is the nozzle partition exit angle measured from tangential taken atRi (see FIG. 2).

While the dimension a should not be less than the minimum axial spacinggiven by this expression, in actual machines, the distance may be madesomewhat greater, depending on an analysis of particle density, particlesize, and other variables.

The operation of the erosion protective arrangement may be described asfollows. As the motive fluid is deected into a tangential direction bynozzle partitions 3, both the gas and any entrained solid particlescontained therein are subjected to a strong centrifugal field. The solidparticles, being heavier, move both radially and circumferentially and,provided the Spacing a is at least the minimum distance given by ltheformula above, the particles will exit through annular extraction port17 into the annular collecting chamber 18, and from there will be blownthrough pipe 19 to the separator 2b. The return line 21 will return themotive fluid to a restricted passageway 22, which is designed to controlthe amount of motive uid being bled off through part 19. Of course,other means such as orices or control valves may be substituted for themeans to control the bleed flow, inasmuch as this should be held to aslow a ligure as possible. It is expected that a bleed fiow through line19 on the order of 3% of the total gas ilow will effectively remove theily-ash.

The spacing a introduces certain aerodynamic losses which are related tothe boundary action of stationary walls on the axially andcircumferentially moving gas stream. These losses may be greatly reducedby the provision of a specially rotating inner side Wall formed by thedummy buckets 16. Since the inner side wall of the flow duct is turningin the same direction as the gas stream, the aerodynamic losses whichwould otherwise occur through friction from the boundary layer will bereduced by the use of such a rotating inner side wall.

These losses may be further reduced by increasing the number of stagesin the turbine. Ordinarily, in a gas turbine designed to burn naturalgases or residual oil, the irst stage nozzle velocity is very high,perhaps on the order of 2000 ft./sec. when only two turbine stages areused. In a coal-burning gas turbine ,on the other hand, a greater numberof stages, tive stages in the embodiment shown here, provide the dualfunction of reducing the gas velocity and also of reducing the overallaerodynamic losses due to gap a. As to the rst reason for increasing thenumber of stages, the erosion which takes place is an exponentialfunction of the velocity of the gas relative to the blades. It has beendetermined that, for satisfactory blade life, these relative velocitiesmust be kept below 900 ft./sec. and preferably below 500 t./sec. Byusing a larger number of stages, the pressure drop per stage, hence thegas velocity, can be reduced. The second reason for increasing thenumber of stages is that a certain extra percentage of aerodynamic lossis associated with each stage employing a spacing such as (1. The lossattributed to only the first stage spacing can be reduced insofar as thewhole turbine is concerned by allotting a smaller portion of the totalwork to the first stage. Therefore, the total loss due to spacing a isprogressively reduced for the overall turbine as stages are successivelyadded. Thus, for most effective practice of the invention, it isanticipated that at least four stages are desirable, and preferably tiveor six stages for the optimum results.

Although the embodiment shown in FIG. l provides for actual removal ofthe ily-ash from the motive fluid, it is lalso within the purview of theinvention to utilize the critical spacing a, to control erosion byemploying this critical spacing between a nozzle and its cooperatingbucket wheel, without using extraction part 17 and collecting chamber1S. In this case, the minimum spacing a would be that required betweenthe trailing edge 3d ot tbe nozzle partition 3 and the leading edge ofthe adjacent bucket 9. By utilizing this minimum spacing between nozzleand bucket, any entrained solid particles will move to the outermostwall of the flow annulus before striking any part of the first-stagebucket. Ilhereafter the particles pass between the bucket tip and thecasing through the clearance space 25, in which passage the particleswill be re-accelerated axially due to the increased gas velocityoccasioned by the pressure drop occurring across the bucket tips. Theextent of erosion damage is minimized' by impingement of these particleson the erosion-resistant inserts 15 in the second-stage nozzle base 4bprojecting into the gas path. The hardened erosion shields 15 aredescribed in more detail in the aforementioned U. S. Patent 3,030,071. Asimilar action occurs in the clearance spaces of the following stageswhich are similarly provided with erosion-resistant inserts. Thus by useof centrifuging spacing a without using extraction part 17 the erosiveparticles are concentrated at the outer sidewall where they will tend toremain throughout the entire turbine, and their erosion damage will belimited to an area near the bucket tips and primarily on the erosionshields 15, rather than in the more critical area of the highly stressedbucket roots.

It can be seen that the erosion protective arrangement described isprimarily to protect the buckets of the early stages where the erosionproblem is the most critical due to higher temperatures. The use of arotating sidewall in the gap a and the use of at least four stagesreduce the aerodynamic losses occasioned by the introduction of theaxial spacing to an acceptable amount.

Therefore, the invention provides -an effective arrangement forprotecting the `most highly stressed members in a coal-burning gasturbine against erosion by entrained yash. The same principle is alsoobviously applicable to other turbo machines, such as steam turbinesoperating on saturated steam, in which the motive fluid contains solidor liquid particles which can be centrifuged and removed in the mannerdescriber herein. It is of course intended to cover in the appendedclaims all such modications and applications as fall within the truespirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. In a turbomachine utilizing uid containing erosive particles :andhaving a plurality of stages of alternating stationary and moving bladesdisposed in a flow annulus symmetrical with respect to the turbomachineaxis, the combination of an outer annular casing portion, acircumferential row of radially extending stationary blades disposed insaid casing portion, an adjacent circumferential row of radiallyextending moving blades, an inner sidewall for said ow annulus disposedbetween said stationary blade row and said moving blade row and rotatingat the same speed as the moving blade row, and annular passage meansdened between the casing portion and the tips of said moving blades, theentrance to said passage means 'being spaced from the trailing edges ofthe stationary blades by a minimum unobstructed axial distance cz=Ri tanaVOEZTP-l where Ri is the inner radius of tire iiow annulus at thestationary blade row measured from the turbomachine axis,

RO is the outer radius of the flow annulus at the entrance to thepassage means measured from the turbomachine axis, and

ori is the angle from tangential at the nozzle partition railing edgemeasured at R1.

2. In a turbomachine utilizing motive fluid containing erosion-producingparticles and having an outer casing portion enclosing a plurality ofstages of alternating nozzle partitions and buckets, said casing, saidnozzle partitions and said buckets delining portions or a iiow annulusfor the `motive fluid, the combination of a stationary circumferentialrow of radially extending firststage nozzle partitions, a movingcircumferential row of radially extending first-stage turbine bucketsspaced a substantial axial distance beyond said nozzle partitions, aninner circumferential sidewall extending between said rst stage nozzlepartitions and said first stage buckets and rotating at the same speedas the buckets, and extraction conduit means defined by the casingportion between the first-stage nozzle partitions and therststage'buckets, and spaced beyond the nozzle partition trailing edgesby a minimum unobstructed axial distance where Ri is the inner radius ofthe fiow annulus at the first stage nozzle partition measured from theturbomachine axis Ru is the outer radius of the flow annulus at theextraction conduit means measured from the turbomachine axis, and

ai is the angle from tangential at the nozzle partition trailing edge,measured at Ri.

3. In `a coal-burning gas turbine, the combination of an outer casingdefining the outer wall of a flow annulus, a plurality of axially-spacedcircumferential rows of nozzle partitions disposed in said casing sidewall and extending radially inward, a rotor turning within said casinghaving circumferentially extending portions dening an inner side wallfor said flow annulus, a plurality of axially spaced circumferentialrows of turbine buckets disposed on said rotor and extending radiallyoutward yand between said spaced nozzle partitions, the first row ofnozzle partitions and the rst row of turbine buckets being spaced apartby a substantial axial distance, an inner circumferential sidewallrotating at the same speed as the first bucket row and extending axiallytherefrom to the first nozzle partition row, said casing yalso detningan annular extraction conduit located between the first nozzle partitionand the rst turbine bucket rows, the entrance to said extraction conduitbeing axially spaced beyond the first nozzle partition trailing edges bya minimum distance where R1 is the inner radius of the flow annulus atnozzle, taken from the rotor axis,

R is the outer radius of the ow annulus at the extraction conduit, takenfrom the rotor axis,

a, is the angle from tangential at the nozzle partition trailing edgetaken at R1.

4. The combination according to claim 3, including means for bleedingmotive uid from said extraction conduit, removing solid particles, andreturning the clean motive fluid to a lower pressure location in themotive iluid ow path in the turbine.

the rst stage 5. in a coal-burning gas turbine having a plurality ofstages of alternating nozzle partitions and buckets disposed in a flowannulus symmetrical with respect to the turbine axis, the combination ofa circumferential row of radially extending first stage nozzlepartitions, a casing portion defining a circumferential outer sidewallfor a ow annulus, a moving circumferential row of radially extendingfirst-stage turbine Ibuckets having tips radially spaced from saidcasing portion, annular pass-age means defined between the casingportion and the bucket tips, the entrance to said passage means beingaxially spaced from the nozzle partition trailing edges by a minimumdistance where Ri is the inner radius of the flow annulus at the firststage nozzle partition measured from the turbine axis,

R0 is the outer radius of the flow annulus at the entrance to thepassage means measured from the turbine axis,

ai is the angle from tangential trailing edge measured at R1,

at the nozzle partition inner circumferential sidewall extending betweensaid iirst stage nozzle partitions and said rst stage buckets androtating at the same speed as the buckets, and an annular impingementwall extending radially inward from said casing portion on the otherside of said passage means from said space a, whereby ily ash particlesleaving said nozzle partitions move to the outer sidewall beforereaching said ybuckets and, after passing thro-ugh said passage means,are decelerated by striking said annular impingement wall.

6. In ya turbomachine utilizing fluid containing erosive particles andhaving a plurality of stages of alternating, stationary and movingblades disposed in a flow annulus symmetrical with respect to theturbomachine axis, the combination of an outer annular casing portion, acircumferential row of radially-extending stationary blades disposed insaid casing portion, an axially spaced, cooperating circumferential rowof radially extending moving blades, an inner circumferential sidewallextending axially between said moving blade row and said stationaryblade row and rotating at the same speed as the moving blade row, andannular passage means bypassing said moving blades, the entrance to saidpassage means being located near the tips of said moving blades andbeing spaced fro-m the trailing edges of the stationary blades by aminimum unobstructed axial distance R1 is the inner radius of the flowannulus at the stationary blade row measured from the turbomachine axis,

Ro is the outer radius of the ow annulus at the entrance to the passagemeans measured from the turbomachine axis, and

a, is the angle from tangential at the nozzle partition `trailing edgemeasured at Ri.

References Cited in the le of this patent UNITED STATES PATENTS1,834,452 lFrey Dec. 1, 1931 1,944,520 Meyer Jan. 23, 1934 2,288,734Noack July 7, 1942 2,636,666 Lombard Apr. 28, 1953 2,802,618 PracharAug. 13, 1957 2,879,936 Taught Mar. 31, 1959 FOREIGN PATENTS 547,332Germany Mar. 10, 1932 446,476 Italy Mar. 18, 1949 1,115,125 France Apr.19, 1956

